The present invention relates to a semiconductor device for generating complementary PWM signals for, for example, controlling an inverter, and more particularly, it relates to a technique to add dead times to complementary PWM signals.
An inverter circuit 5 including serially connected two switching elements 51 and 52 as shown in FIG. 11 is widely used as a circuit included in an induction heating apparatus such as an IH cooking device (see Japanese Laid-Open Patent Publication No. 10-149876 (hereinafter referred to as Document 1)). Also, an inverter circuit for driving a motor has an architecture in which three combinations for three phases each of serially connected two switching elements are connected in parallel, and this architecture is basically similar to that of the inverter circuit 5.
Such an inverter circuit is generally controlled in accordance with PWM (Pulse Width Modulation) signals having a time at which none of the phases are simultaneously in an ON state (i.e., a dead time). An example of such signals is shown in FIG. 12A. One of the roles of the dead time is preventing an inverter control circuit from being broken by a through current passing when two switching elements are simultaneously in an ON state as shown in FIG. 12B. Also, in general, the dead time is set in order to minimize power loss of switching elements by setting optimum switching timings in accordance with the on-off characteristics of two different switching elements as shown in FIG. 13. The power loss is obtained as a product of a current and a voltage attained in on-off switching.
Conventionally, the following three methods are known for setting a dead time: 1) General PWM outputs are used and a dead time is added on a control circuit; 2) PWM outputs with a given dead time supplied by a semiconductor device are used; and 3) the methods 1) and 2) are combined.
The method 1) is described in, for example, Document 1, and a dead time can be set by adding a control circuit to an inverter driving circuit for controlling an induction heating cooking device. FIG. 11 described above shows a basic circuit configuration employed in this case, in which a dead time setting circuit 7 generates a signal for simultaneously turning off the two switching elements 51 and 52 by a driving control circuit 6. As shown in FIG. 13, a dead time td1 is set as a period elapsing from time when Vge2 becomes 0 V and the switching element 52 is turned off until time when the residual voltage of Vce1 is minimized, and a dead time td2 is set as a period elapsing from time when Vge1 becomes 0 V and the switching element 51 is turned off until the minus current of Ic2 (i.e., a freewheel diode current included in the switching element 51) is substantially halved. Both the dead times are determined in accordance with a constant of the circuit.
Apart from the aforementioned methods, a method by using a combination of a PWM output and a delay circuit is known. For example, as shown in FIG. 14, dead times are added by providing delays to respective PWM signals by CR circuits 53 and 54 arbitrarily set. Thus, two kinds of output signals with dead times as shown in FIG. 15 can be generated.
Alternatively, for avoiding necessity for adding a control circuit, a microcomputer having a function to generate PWM outputs with dead times has been realized. In setting a dead time in this case, one dead time register is used for setting a dead time, so as to add the same dead time at on-timing and off-timing of the PWM signal.
Many of such microcomputers for controlling an inverter include a counter and a comparator. The counter counts up from 0 to a frequency set value, during which the count value and a duty set value are compared with each other. When the count value and the duty set value accord with each other, an output signal is inverted so as to generate a reference PWM signal. At this point, a dead time is added by comparing the count value and a dead time set value with each other and delaying the on-timing from the inverting timing until they accord with each other. In this case, the dead time can be set as a given period not depending upon the characteristic of a switching element.
However, in the method described in Document 1 or in the method using the CR circuits, the dead time to be set is determined by hardware on a substrate, and therefore, merely a predetermined value can be set for each switching element. Therefore, it is difficult to change/set the optimum value of a dead time, and in order to attain advanced control, for example, it is necessary to provide a control circuit part for changing CR components by hardware. Furthermore, it is difficult to perform precise control because of the waveform of the resultant PWM signal having a time constant and owing to variation in components.
Furthermore, in the method for obtaining a PWM output with a dead time by using the microcomputer for controlling an inverter, since merely one dead time register is used, merely the common dead time can be added at the rise and the fall. This does not cause any particular problem in controlling an inverter circuit in which pairing switching elements have symmetrical characteristics. However, in controlling an inverter circuit in which pairing switching elements have different characteristics, it is desired to individually set optimum dead times, and hence, this method is difficult to employ in such a case.